1. Field of the Invention
The present invention relates to a method for producing a multilayer ceramic substrate, and more particularly, a method for producing a multilayer ceramic substrate by applying a so-called non-shrinkage process.
2. Description of the Related Art
As described in, for example, Japanese Patent Application Laid-Open No. 4-243978, in a method for producing a multilayer ceramic substrate by applying a so-called non-shrinkage process, an unfired multilayer ceramic substrate is prepared which is composed of a plurality of unfired and stacked base material layers containing a low-temperature sintering ceramic material as their main constituent, and constraining layers containing, as their main constituent, a sintering-resistant ceramic powder that is not substantially sintered at the sintering temperature of the low-temperature sintering ceramic material mentioned above are placed on, for example, both principal surfaces of the unfired multilayer ceramic substrate.
Then, the unfired composite laminate body composed of the unfired multilayer ceramic substrate and constraining layers described above is subjected to firing at the sintering temperature of the low-temperature sintering ceramic material. This firing provides a sintered multilayer ceramic substrate sandwiched between the constraining layers.
In the firing step, the sintering-resistant ceramic powder contained in the constraining layers is not substantially sintered, and the constraining layers thus have no substantial shrinkage caused. For this reason, the constraining layers constrain the base material layers, and thus, the base material layers are substantially shrunk only in the thickness direction, while the shrinkage of the base material layers is suppressed in the principal surface direction. As a result, the obtained multilayer ceramic substrate is less likely to undergo non-uniform deformation, and the accuracy can be improved for the shape and dimensions of the multilayer ceramic substrate in the planar direction.
Next, the constraining layers described above are removed, thereby extracting the desired multilayer ceramic substrate. In this case, the constraining layers can be removed easily because of their porous characteristics.
In the non-shrinkage process as described above, it is known that reactive layers are produced at the interfaces between the base material layers and the constraining layers. The reactive layer refers to a layer produced by upward penetration of glass included in the base material layer toward the constraining layer and chemical reaction of the glass with an inorganic material constituting the constraining layer. The reactive layer is produced by the chemical reaction, thus less likely to be removed as compared with the porous constraining layers, and may be slightly left on the obtained multilayer ceramic substrate. In particular, if the reactive layer is left on a surface electrode, the platability and wire bonding property with respect to the surface electrode will be decreased, thereby leading to a problem that defective mounting is more likely to be caused. Thus, it is desired that the upward penetration of the glass will be suppressed to prevent the production of a reactive layer.
In this regard, for example, Japanese Patent Application Laid-Open No. 6-171976 and Japanese Patent Application Laid-Open No. 2001-114556 disclose the capability to achieve a stable crystallization temperature by the addition of a seed crystal to the entire base material layer.
Thus, when the technique described in these Japanese Patent Application Laid-Open No. 6-171976 and Japanese Patent Application Laid-Open No. 2001-114556 is applied to the non-shrinkage process described in Japanese Patent Application Laid-Open No. 4-243978 mentioned above, the crystallization temperature at the base material layer can be lowered by the addition of a seed crystal. This lowered crystallization temperature can make an adjustment so as to reduce upward penetration of glass from the base material layer to the constraining layer, and suppress the production of a reactive layer to some extent. This is because the upward penetration of the glass is affected by the amount of glass flowing from the softening temperature of the glass to the crystallization temperature thereof.
However, when the technique of adding a seed crystal in an equal amount to the entire base material layer as described in Japanese Patent Application Laid-Open No. 6-171976 and Japanese Patent Application Laid-Open No. 2001-114556 is applied to the non-shrinkage process described in Japanese Patent Application Laid-Open No. 4-243978, the following problem may be caused.
On at least one principal surface of a multilayer ceramic substrate, typically, surface electrodes are formed. As for the total area of the surface electrodes, the total area on one principal surface side of the multilayer ceramic substrate and the total area on the other principal surface side thereof are rarely equal to each other. When the total area of the surface electrodes on one principal surface side of the multilayer ceramic substrate and the total area of the surface electrodes on the other principal surface side thereof are different from each other, the result is that the one and the other principal surfaces of the multilayer ceramic substrate will have different periods of time from each other from the softening temperature to the crystallization temperature. More specifically, the result is that the one and the other principal surfaces of the multilayer ceramic substrate will have the same crystallization temperature, but have different softening temperatures from each other. This is because the metal diffuses during firing to act to lower the softening temperature of the glass included in the base material layer in the vicinity of the surface electrodes, thus resulting in the softening temperature being lowered more on the principal surface side with the larger total area of the surface electrodes.
Thus, when the total area of surface electrodes is, for example, smaller on a first principal surface side and larger on a second principal surface side, the crystallization temperature will be adjusted depending on the second principal surface with the larger total area of the surface electrodes, in order to suppress the production of a reactive layer. Then, however, at the first principal surface with the smaller total area of the surface electrodes, the period of time from the softening temperature to the crystallization temperature will be excessively shortened to start crystallization before densification is completed, thereby resulting in the production of a porous base material layer with a lot of pores on the first principal surface side. This condition not only will decrease the plating resistance of the base material layer which provides the first principal surface, but also may cause the ingress of a plating solution into the pores to adversely affect the electrical characteristics.
It is to be noted that the fact that the total area of surface electrodes on one principal surface side of a multilayer ceramic substrate and the total area of surface electrodes on the other principal surface side thereof are different from each other includes cases in which surface electrodes are formed only on one principal surface of a multilayer ceramic substrate, but not formed on the other principal surface thereof.